1. Field of the Invention
The present invention generally relates to image processing, and more particularly to image contrast enhancement without substantial noise boost.
2. Description of the Prior Art
As the image perception of a viewer is commonly more sensitive to contrast than absolute values themselves, contrast enhancement is one of several very important image processes utilized to improve the quality of an image. However, as the image is subjected to contrast enhancement, noise in the original image tends disadvantageously to be amplified or boosted at the same time, as shown in FIG. 1A. This hinders the application of the contrast enhancement process in some situations, such as television signals, where noise is typically present.
In order to relieve this problem, noise reduction is performed prior to the contrast enhancement, as shown in FIG. 1B. Nevertheless, conventional noise reduction not only suppresses the noise but also tends to blur image details, thereby degrading the quality of the image with respect to the image details. Kim discloses in U.S. Patent Application Publication No. 2006/0013503 a method of preventing noise boost and image blur when applying image contrast enhancement, the second figure of which is reproduced herein as FIG. 2 and the entire disclosure of which is hereby incorporated by reference.
Referring to FIG. 2, Kim uses a transform ratio construction block 38 to compute a transform ratio γ as:γ(x,y)=f(i(x,y))/i(x,y)  (1)where i(x,y) is the (x,y)th pixel value, and f represents a contrast enhancement function constructed in block 34.
The division operation in the expression (1) is performed by a transform ratio construction block 38 or a divider, the implementation of which requires a large gate count or large number of transistors (to implement the divider). As the gate count is one important factor in deciding the overall cost, it is thus impractical to use Kim's transform ratio construction block 38 in products such as consumer electronic products.
Moreover, Kim needs a reserved memory 32 or buffer to compute a locally smoothed transform ratio by low pass filtering the transform ratios of the input pixels in a local window Wp(x,y). The locally smoothed transform ratio out of block 38 is then multiplied with the input pixel i(x,y) from the reserved memory 32. The required use of this reserved memory 32 adds to the overall cost.
For the reason that such a conventional method or system cannot effectively and, more particularly, economically suppress the noise-boost effect when applying contrast enhancement on images, a need has arisen to propose a novel system and method of image contrast enhancement that is not only capable of suppressing noise boost but also can facilitate economic implementation.